The present invention relates to semiconductor wafer chucks in general, and, more particularly, a non-planar semiconductor wafer chuck and method of using the same.
Manual and automatic mask aligment systems are well known in the semiconductor industry. A representive example of an alignment system is the Model CA-400 mask alignment system manufactured by the Cobilt Division of The Computervision Corporation, Sunneyvale, Calif. In mask alignment systems, a semiconductor wafer is either manually or automatically aligned with an overlying photomask for subsequent exposure of the photosensitive wafer surface. The wafer is positioned on a wafer chuck which moves the semiconductor wafer into contact with the photomask. During exposure, the wafer and mask are held in firm contact by a vacuum while a low-pressure nitrogen "pillow" is applied to the back side of the wafer. A nitrogen purge is present in the area over the wafer during the alignment and surrounding the wafer during mask and wafer contact.
Conventional wafer chuck systems normally utilize a leveling device to position the semiconductor wafer in a plane parallel to the plane of the photomask. If all semiconductor wafers were uniformly planar, such a system would provide uniform contact of the wafer against the surface of the photomask. However, in actuality the semiconductor wafers do not have a uniform planar surface. Many wafers have a "dished" configuration so that when the wafers are pressed against the overlying photomask, only the peripheral edge of the wafer contacts the photomask. The peripheral contact pattern of the dished semiconductor wafer provides a circular seal around the wafer which traps a portion of the nitrogen purge within the dished area of the wafer between the wafer and the overlying photomask. When pressure is applied to the back surface of the wafer, the entrapped nitrogen provents a uniform planar contact between the semiconductor wafer and the photomask.
It is accordingly a general object of the present invention to provide an improved wafer chuck and method of using same.
It is a specific object of the present invention to provide a non-planar, semiconductor wafer chuck and method of using the same which produces a uniform, planar contact between a non-planar semiconductor wafer and a photomask.